Members of the Fire Brigades Union (FBU) picket Cambridge fire station during their last strike on Saturday. Photograph: Chris Radburn/PA

Firefighters are to strike on Christmas Eve and New Year's Eve and will refuse to work voluntary overtime during the weekend after Christmas, in an escalation of their long-running dispute with the government over pensions.

Members of the Fire Brigades Union (FBU) in England and Wales will walk out for five hours from 7pm on 24 December, for six hours from 6.30pm on 31 December and for two hours from 6.30am on 3 January.

FBU members in Scotland will take part in the industrial action for the first time since the row flared, by joining the ban on overtime. Between 7pm on 27 December and 7pm on the 29th, all firefighters in the union across England, Wales and Scotland will refuse to work voluntary overtime.

With the new walkouts, the union will have staged nine separate strikes since September. The FBU's general secretary, Matt Wrack, said: "Firefighters provide a first-class standard of service 24 hours a day, 365 days a year, and these strikes will remind government just how reliant they are on our members' professionalism, commitment and flexibility.

"However, there should be no need for industrial action, and it's absurd that firefighters' concerns over pensions have not been addressed already. The government must stop claiming they are negotiating when they have refused to talk for two months and insist on forcing through proposals that are unaffordable, unworkable and unfair. By simply conceding common sense and allowing firefighters a fair deal, the government could end this industrial action today."

The union argues that firefighters in their late 50s face the prospect of being sacked or seeing their pension reduced by almost half because of changes in the retirement age.

Negotiations are ongoing in Scotland and have made enough progress to avert strikes, but the union said no settlement had been made.

The FBU said firefighters' pension schemes were among the most expensive for workers anywhere in the public or private sector, but among the cheapest proportionally for the government.

Firefighters take home around £1,650 a month. Most pay £320 or more a month into their pensions, and from next April this would rise for the third year in a row, to more than £340 a month or £4,080 a year. The FBU said many faced a fourth consecutive rise of 2.2% in 2015.

The union claims that the government's proposals are "designed to fail" because they ignore the physical demands and fitness standards required of firefighters. "Evidence suggests that at least two-thirds of the current workforce will face dismissal or their pension reduced by almost half because they are unable to maintain the fitness standards required by the fire service beyond the age of 55," the union said.

Investigations are continuing into the death of a man in a fire in London that broke out on Saturday evening during the last strike.